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<div class="chapter">
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<div><h2 class="title">
<a name="lambda"></a>Chapter 17. Boost.Lambda</h2></div>
<div><div class="author">
<h3 class="author">
<span class="firstname">Jaakko</span> <span class="surname">Järvi</span>
</h3>
<code class="email">&lt;<a class="email" href="mailto:jarvi%20at%20cs%20tamu%20edu">jarvi at cs tamu edu</a>&gt;</code>
</div></div>
<div><p class="copyright">Copyright © 1999-2004 Jaakko Järvi, Gary Powell</p></div>
<div><div class="legalnotice">
<a name="id-1.3.18.1.3"></a><p>Use, modification and distribution is subject to the Boost
    Software License, Version 1.0. (See accompanying file
    <code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="lambda.html#introduction">In a nutshell</a></span></dt>
<dt><span class="section"><a href="lambda/getting_started.html">Getting Started</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/getting_started.html#id-1.3.18.4.2">Installing the library</a></span></dt>
<dt><span class="section"><a href="lambda/getting_started.html#id-1.3.18.4.3">Conventions used in this document</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/s03.html">Introduction</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/s03.html#id-1.3.18.5.2">Motivation</a></span></dt>
<dt><span class="section"><a href="lambda/s03.html#id-1.3.18.5.3">Introduction to lambda expressions</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/using_library.html">Using the library</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/using_library.html#lambda.introductory_examples">Introductory Examples</a></span></dt>
<dt><span class="section"><a href="lambda/using_library.html#lambda.parameter_and_return_types">Parameter and return types of lambda functors</a></span></dt>
<dt><span class="section"><a href="lambda/using_library.html#lambda.actual_arguments_to_lambda_functors">About actual arguments to lambda functors</a></span></dt>
<dt><span class="section"><a href="lambda/using_library.html#lambda.storing_bound_arguments">Storing bound arguments in lambda functions</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/le_in_details.html">Lambda expressions in details</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.placeholders">Placeholders</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.operator_expressions">Operator expressions</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.bind_expressions">Bind expressions</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.overriding_deduced_return_type">Overriding the deduced return type</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.delaying_constants_and_variables">Delaying constants and variables</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.lambda_expressions_for_control_structures">Lambda expressions for control structures</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.exceptions">Exceptions</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.construction_and_destruction">Construction and destruction</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#id-1.3.18.7.11">Special lambda expressions</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#id-1.3.18.7.12">Casts, sizeof and typeid</a></span></dt>
<dt><span class="section"><a href="lambda/le_in_details.html#lambda.nested_stl_algorithms">Nesting STL algorithm invocations</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/extending.html">Extending return type deduction system</a></span></dt>
<dt><span class="section"><a href="lambda/s07.html">Practical considerations</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/s07.html#id-1.3.18.9.2">Performance</a></span></dt>
<dt><span class="section"><a href="lambda/s07.html#id-1.3.18.9.3">About compiling</a></span></dt>
<dt><span class="section"><a href="lambda/s07.html#id-1.3.18.9.4">Portability</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/s08.html">Relation to other Boost libraries</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="lambda/s08.html#id-1.3.18.10.2">Boost Function</a></span></dt>
<dt><span class="section"><a href="lambda/s08.html#id-1.3.18.10.3">Boost Bind</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="lambda/s09.html">Contributors</a></span></dt>
<dt><span class="section"><a href="lambda/s10.html">Rationale for some of the design decisions</a></span></dt>
<dd><dl><dt><span class="section"><a href="lambda/s10.html#lambda.why_weak_arity">
Lambda functor arity
</a></span></dt></dl></dd>
<dt><span class="bibliography"><a href="lambda.html#id-1.3.18.13">Bibliography</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="introduction"></a>In a nutshell</h2></div></div></div>
<p>

      The Boost Lambda Library (BLL in the sequel) is a C++ template
      library, which implements a form of <span class="emphasis"><em>lambda abstractions</em></span> for C++.
The term originates from functional programming and lambda calculus, where a lambda abstraction defines an unnamed function.
      The primary motivation for the BLL is to provide flexible and
      convenient means to define unnamed function objects for STL algorithms.
In explaining what the library is about, a line of code says more than a thousand words; the
      following line outputs the elements of some STL container
      <code class="literal">a</code> separated by spaces:

      </p>
<pre class="programlisting">for_each(a.begin(), a.end(), std::cout &lt;&lt; _1 &lt;&lt; ' ');</pre>
<p>

      The expression <code class="literal">std::cout &lt;&lt; _1 &lt;&lt; ' '</code> defines a unary function object. 
      The variable <code class="literal">_1</code> is the parameter of this function, a <span class="emphasis"><em>placeholder</em></span> for the actual argument. 
      Within each iteration of <code class="literal">for_each</code>, the function is
      called with an element of <code class="literal">a</code> as the actual argument.
      This actual argument is substituted for the placeholder, and the <span class="quote">“<span class="quote">body</span>”</span> of the function is evaluated.
    </p>
<p>The essence of BLL is letting you define small unnamed function objects, such as the one above, directly on the call site of an STL algorithm.
    </p>
</div>
<div class="bibliography">
<div class="titlepage"><div><div><h2 class="title">
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